There are two options for modeling an Air Source Heat Pump with a fossil fuel backup system (Dual Fuel Heat Pump):
- (Simplest) Simply model one ASHP system with the proper HSPF and SEER ratings, and do not model the fuel backup system. Ekotrope already assumes that some energy will be required as backup heat. However, Ekotrope will assume this is electric energy, so you will not see any fossil fuel use for heating in Ekotrope reports.
- (Most HERS Credit) If you need to distinguish between fossil fuel use and electric use, you can model the system as two systems instead. Model one ASHP with the proper HSPF and SEER ratings, and set the "% Heating Load" to the % time that you expect the system will run without fossil fuel backup (probably somewhere between 80% and 95%). Then model a Furnace with the proper AFUE and set the "% Heating Load" to the leftover load (5% - 20%). You will also need to create two identical distributions systems with the proper inputs, and link one to each heating system. In order to prevent Ekotrope from derating the ASHP for cold-climate effects and resistance heat backup, you can also change the efficiency type of the heat pump to "COP" and set it to HSPF / 3.412.
If you have further questions, please contact Ekotrope support at firstname.lastname@example.org